Dual Chemical Role of Ag as an Additive in Chalcogenide Glasses

Abstract

Ternary $({\mathrm{Ge}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}{)}_{1\ensuremath{-}y}{\mathrm{Ag}}_{y}$ bulk glasses in the Se-rich region $(x<\frac{1}{3})$ are shown to be intrinsically phase separated into an ${\mathrm{Ag}}_{2}\mathrm{Se}$-rich glass and a residual ${\mathrm{Ge}}_{t}{\mathrm{Se}}_{1\ensuremath{-}t}$ backbone ( $t>x$ at $y\ensuremath{\ne}0$) with Ag acting as a network modifier. The existence of an ${\mathrm{Ag}}_{2}\mathrm{Se}$ glass with a ${T}_{g}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}230\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ is explained quantitatively in terms of constraint counting algorithms. In contrast, Ge-rich glasses $(x\ensuremath{\ge}\frac{2}{5})$ are homogeneous, wherein Ag acts as a network former, replacing available Ge sites of the backbone to be 3-fold coordinated to Se.